- Getting people to donate in a recurring fashion is exceedingly hard. Not impossible (as some patreons prove), but still hard.
- Getting businesses to donate at all, let alone in a recurring fashion, is even harder.
mdn needs money, not just for content, but to keep the lights on. Their content is furthermore aimed largely at "professionals" (and some enthusiasts), meaning convincing businesses to give money is even more important than e.g. is the case with wikipedia or your random youtube content creator.
Businesses are easier to convince to spend money if you offer them something in return. Doesn't really have to be much or something particularly valuable, just something, anything really, that then can be used to justify the expense to management/comptrollers/legal/owners as a "valid" expense.
I personally had people contact me in the past, on more than one occasion, saying they made good use of some code I open sourced in their commercial stuff, and they'd like to gift me something, but they cannot get permission from their employer to transfer any funds unless I formally enter a "consulting" contract (and NDA and yadayada) or officially sell them something. So the best they could do is offer me some company swag and/or a small donation out of their own pocket. So now I own a bunch of T-Shirts and coffee mugs from various companies :P (and I am OK with that, since I never had the intention to profit from that code).
So creating some easy "premium features" may indeed enable mdn to collect more money, especially from businesses, compared to them just asking for donations.
It remains to be seen if that will work for mdn, and if mdn will then use the money "wisely", but I really cannot fault them for their approach so far...